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Pushing Beyond Pluto

What would you do if you had a spacecraft pushing toward the edge of the Solar System with nothing much to do? The answer is to assign it an extended mission, as we found out with the two Voyagers and their continuing data return that is helping us understand the boundaries of the heliosphere. In the case of New Horizons, NASA’s probe to Pluto/Charon, two extended missions may be involved after Pluto, the first being a flyby of one or more Kuiper Belt targets, the second being a further look at what is actually going on where the solar wind meets the interstellar medium.

Alan Stern, principal investigator for New Horizons, comments on the possibility in his latest report on the mission, noting that a second extended mission isn’t out of the question, and adding that New Horizons won’t make it as far as the Voyagers before it runs out of power. But 90 to 100 AU seems a possibility, which would provide a useful supplement to Voyager data. Remember that New Horizons carries two instruments ideal for this part of the system. The first is the Solar Wind Around Pluto (SWAP) plasma instrument, the second the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI). All this is in addition to what the spacecraft’s dust counter, its two imagers and its ultraviolet spectrometer may tell us.

Stern’s report on New Horizons comes at the same time we have word, from a new paper in Science, that the assumed ‘bow shock’ at the outer edge of the heliosphere may not actually exist. The bow shock is at the boundary between the solar wind pushing out from the Sun and the interstellar medium, an area of presumed turbulence that has been observed around other stars. The new paper from David McComas (SwRI) and colleagues presents findings from the Interstellar Boundary Explorer (IBEX) that show the Sun is moving more slowly in relation to the interstellar medium than previously thought, slow enough to prevent a bow shock from forming.

Image: IBEX has caught the interstellar wind that surrounds and compresses our heliosphere and has found that it travels more slowly and in a different direction than previously thought. This new understanding has important implications for the size and shape of the heliosphere and may inform the history and future of the solar system. Credit: SwRI.

Maybe New Horizons can help us clarify the situation with studies of the outer heliosphere, but we still lack a mission that could get us out as far as the bow shock region itself (the Voyagers have entered the heliosheath, but their signals will surely be lost before getting to the needed 200 AU or more from the Sun). All of this ties in with recent Cassini results suggesting that the heliosphere is more spherical than comet-shaped, so perhaps the interactions at system’s edge aren’t quite as fierce as has been thought. We need Innovative Interstellar Explorer to learn more, or a comparable mission specifically designed to penetrate into true interstellar space.

New Horizons in the Kuiper Belt

Meanwhile, our Pluto/Charon mission is, says Stern, doing just fine, having exited a period of hibernation on April 30 to begin a series of extensive systems checkouts. The spacecraft’s Pluto encounter occurs in the summer of 2015, but it should take a year to get all the encounter data back to Earth due to the slow data transmission rates at that distance. It’s after that that the first extended mission, subject to approval by NASA, would study objects in the Kuiper Belt. The spacecraft should have about 40 percent of its fuel still available, so a choice of KBOs should be possible, assuming the ongoing hunt for likely candidates turns up workable targets.

The New Horizons team has been using Earth-based telescopes to hunt for KBOs, but so far none has been identified that would be within range of New Horizons. It’s a tricky search, and one Stern assumes will succeed, but his recent report explains some of the problems:

First, the only KBOs within our reach are likely to be small, roughly 50 kilometers in diameter. Because they are small and far away, they will be faint as seen from Earth. In fact, calculations show that the KBOs we need to find are going to be about 25,000 times fainter than Pluto, which is itself about 10,000 times fainter than the eye can see. This means we have to search for objects with the largest telescopes and most sensitive astronomical cameras on Earth.

The second factor making the search tough is that our trajectory is pointed at the heart of the Milky Way’s densest star fields — those of the galactic center in the constellation Sagittarius. So our search is kind of a “needle in a haystack” hunt for very faint objects slowly moving against regions of the sky thick with stars!

All I could think of when I read that was an image of Clyde Tombaugh working the blink comparator at Lowell Observatory in 1930. I wonder what he would have thought then of the chances for tracking a target 25,000 times fainter than the dim planet (well, dwarf planet) he eventually found. The New Horizons effort should have at least one target defined by 2015, at which time an engine burn in the fall of that year would change the spacecraft’s trajectory to reach the first KBO, a journey that — if the team’s calculations are accurate — should last three or four years and perhaps longer. That could place the first KBO flyby as early as 2018 or as late as 2021.

What’s exciting about Stern’s report this time around is his statement that any target KBO will be approached at distances perhaps as close as the Pluto/Charon flyby, which means we should get images from the KBOs that are as detailed as those from Pluto. The down side: With no other Kuiper Belt mission in the works, we’ll need every bit of New Horizons’ observations on KBO surface composition and features, temperatures, moon or rings system and anything else the brief encounter can deliver, for as Stern puts it, “New Horizons is very likely to be the only spacecraft that will explore KBOs in the lifetime of most people alive today.”

Or maybe not. Let’s assume that first extended mission into the Kuiper Belt will be approved, which will yield not only close-up KBO images but also more distant observations of KBO satellites as well as dust particle distribution data. And while we get behind missions like Innovative Interstellar Explorer and push to see them implemented, let’s also cross our fingers for that second extended mission that would keep New Horizons active until it goes silent somewhere out around 100 AU. As the latest IBEX findings remind us, we know all too little about the boundary between our Solar System and the interstellar gulf beyond.

The new IBEX findings are in McComas et al., “The Heliosphere’s Interstellar Interaction: No Bow Shock,” published online in Science 10 May 2012 (abstract).


Comments on this entry are closed.

  • Daniel May 11, 2012, 14:11

    “but it should take a year to get all the encounter data back to Earth due to the slow data transmission rates at that distance.”

    a year??? but from Pluto distance is it supposed take 6 hours to transmit the data back to Earth at speed of light?

  • Paul Gilster May 11, 2012, 14:30

    Daniel, what Stern means is that it will take a year’s worth of transmitting data to get the entire package back to the Earth because of the slow data rate.

  • Daniel May 11, 2012, 15:51

    All Right,now I got it.

    thanks Paul

  • Interstellar Bill May 11, 2012, 17:18

    Can’t they use New Horizon imagery to pick out a KBO’s motion in front of the Milky Way? It’s not like the imagery team has much to do now but try to find Pluto, so when NH is within detection range of their KBO zone, and its well before the Pluto encounter itself, they should be sending starfield-search pictures back daily, not six months later.

  • Paul Gilster May 11, 2012, 18:19

    One of the problems is that they’re looking right into Sagittarius — at galactic center — not an easy task when it comes to picking out objects as faint as these.

  • Interstellar Bill May 11, 2012, 20:17

    Bring back the blink comparator.

    If the KBO object is above NH’s camera threshold,
    then sooner or later it will have an image
    not overlapped by other stars.
    If it does it three times you just might see it.

  • Andrew W May 11, 2012, 23:33

    Can’t they use New Horizon imagery to pick out a KBO’s motion in front of the Milky Way?

    Even if the rate of data transmission from NH was high enough to make search for KBO’s with imaging systems on the probe feasible, a KBO 20AU from Pluto and 60 AU from Earth will only be 10 times brighter to a telescope at Pluto than it would to the same telescope on Earth, so I expect the light gathering power of the probes imaging systems is far, far too low to search for KBO’s.

  • Bob Andrews May 12, 2012, 2:51

    Paul. Given the restraints in NASA’S future funding are we neglecting the likes of China or India (or even Brazil) when it comes to possible future missions? Perhaps co-operation between States incorporated in a Universal Space Programme is the answer or is this just wishful thinking? Best wishes for your continuing insightful posts.

  • Rafal May 12, 2012, 7:38

    If they don’t find any good KBO candidate, can they use te remaining fuel to speed up and therefore reach further distance? This could, as you pointed in the beginning, supplement our Voyager data on the bounduaries of heliosphere.

  • Paul Gilster May 12, 2012, 8:40

    Bob Andrews: International cooperation is a laudable goal, although it introduces as many problems as it solves, especially in terms of bureaucracy! But it’s hard to see how a sustained effort moving out into the Solar System will happen without a large degree of cooperation, and of course the commercial aspects of all this — via companies like Planetary Resources, etc. — may take us in unexpected directions. Anyway, I don’t see international cooperation as wishful thinking at all, and I hope we see a lot of it once we get into serious infrastructure building.

    Rafal: I think you can count on New Horizons, assuming its mission is extended, to make at least one KBO flyby. There are a vast number of objects out there that should be in range, and I think the Earth-based observations of the region will turn up several strong possibilities. I don’t know of any contingencies planned into the mission if no KBO is found, though.

    Andrew W writes:

    Even if the rate of data transmission from NH was high enough to make search for KBO’s with imaging systems on the probe feasible…

    Exactly so, and what I should have said in the first place. Using New Horizons to pick out its own targets in the Kuiper Belt fails due to data transmission issues. So the hunt for suitable KBOs for New Horizons has to happen here on Earth.

  • Michael May 12, 2012, 13:31

    The camera should be of a quality that will allow parallax imaging for star distances to get accuracy. Pity they could not release the images so we could look for KBO using our PC’s, sometimes I have little to do, in fact most of the time.

  • Kappy May 12, 2012, 16:52


    Such a program already exists, although the images are from ground based telescopes rather than from New Horizons itself. You can participate here: http://www.icehunters.org/

  • djlactin May 13, 2012, 11:56

    Kappy [and Michael]:
    “Icehunters” is now defunct, apparently due to some sort of conflict with the rest of the Zooniverse crowd. The replacement site is
    Brief report on Icehunters: they/we scanned >8,000,000 images (each numerous times) and spotted ca. 78 candidate KBOs, although these were in before/after pairs (and perhaps pairs of pairs; I was not able to determine). So far, they have reported two new KBOs to the minor planet center. Neither has orbital parameters that allow New Horizons to rendezvous with them.

  • ljk May 14, 2012, 9:29

    Fear not, one ounce of Clyde Tombaugh is aboard New Horizons. He will guide the probe to the KBOs.


  • ljk June 15, 2012, 14:51


    Voyager 1 Breaking Through the Borders of the Solar System

    by Jason Major on June 15, 2012

    After almost 35 years traveling at over 35,000 mph, the venerable (and still operational!) Voyager 1 spacecraft is truly breaking through to the other side, crossing the outermost boundaries of our solar system into interstellar space — over 11 billion miles from home.

    Data received from Voyager 1 — a trip that currently takes the information 16 hours and 38 minutes to make — reveal steadily increasing levels of cosmic radiation, indicating that the spacecraft is leaving the relatively protected bubble of the Sun’s influence and venturing into the wild and wooly space beyond.

    From the JPL press release:

    “The laws of physics say that someday Voyager will become the first human-made object to enter interstellar space, but we still do not know exactly when that someday will be,” said Ed Stone, Voyager project scientist at the California Institute of Technology in Pasadena. “The latest data indicate that we are clearly in a new region where things are changing more quickly. It is very exciting. We are approaching the solar system’s frontier.”

    The data making the 16-hour-38 minute, 11.1-billion-mile (17.8-billion-kilometer), journey from Voyager 1 to antennas of NASA’s Deep Space Network on Earth detail the number of charged particles measured by the two High Energy telescopes aboard the 34-year-old spacecraft. These energetic particles were generated when stars in our cosmic neighborhood went supernova.

    “From January 2009 to January 2012, there had been a gradual increase of about 25 percent in the amount of galactic cosmic rays Voyager was encountering,” said Stone. “More recently, we have seen very rapid escalation in that part of the energy spectrum. Beginning on May 7, the cosmic ray hits have increased five percent in a week and nine percent in a month.”

    This marked increase is one of a triad of data sets which need to make significant swings of the needle to indicate a new era in space exploration. The second important measure from the spacecraft’s two telescopes is the intensity of energetic particles generated inside the heliosphere, the bubble of charged particles the sun blows around itself. While there has been a slow decline in the measurements of these energetic particles, they have not dropped off precipitously, which could be expected when Voyager breaks through the solar boundary.

    “When the Voyagers launched in 1977, the space age was all of 20 years old. Many of us on the team dreamed of reaching interstellar space, but we really had no way of knowing how long a journey it would be — or if these two vehicles that we invested so much time and energy in would operate long enough to reach it.”

    – Ed Stone, Voyager project scientist, Caltech

    Read more on the JPL site here.